DNA hybridization techniques of increased sensitivity for the detection of endogenous genes distantly related to viral cDNAs have been developed and applied for three endogenous Old World primate viruses. All three viruses (M28 from baboon, MAC-1 from macaque, and LAD-1 from langur) show different evolutionary patterns in Asian primates as opposed to African primates. All three viruses have some distant homology with the other two viruses and with some mouse viruses. In spite of being able to detect related endogenous sequences in the DNA of some nonprimate species, related sequences can only be detected in some ape species (chimpanzee and gorilla) and not in others (orangutan, gibbon and man). Our efforts to develop probes specific to the most conserved regions of the primate viral genome have shifted to (a) isolating the fractions held in common among the three primate viruses and mouse viruses and (b) generating specific fragments of the viral genome by use of restriction enzymes.